This invention relates to combustion engine driven vehicles and particularly to such vehicles adapted to operate on a mixture of fuels such as gasoline and alcohol in unknown, variable concentrations. The engines of such vehicles are optimally operated with one or more engine operating parameters varied in response to a fuel composition signal generated by a sensor in the engine fuel supply which is responsive to a parameter of the fuel mixture indicating fuel composition.
The fuel composition signal is preferably provided to an engine control computer programmed so as to control the fuel composition affected engine operating parameters in response to the signal. Such an engine control computer is powered from a vehicle electrical power supply which provides a computer analog reference voltage relative to a computer ground voltage. To be most accurate and meaningful, the voltage of the fuel composition signal input to the computer should be referenced to the computer ground voltage and should have a range between its maximum and minimum possible values which is optimally matched to the allowable input voltage range of the computer.
However, the cost of the fuel composition sensor may be significantly reduced if one of the capacitor electrodes is grounded, since that electrode need not be insulated from the vehicle body. If this is done, the sensor ground voltage may differ from the computer ground voltage by as much as a volt. In addition, the sensor has its own power supply providing its own analog reference voltage; and the sensor output voltage may further be compressed or expanded relative to the range of allowable input to the computer. Thus, the sensor output voltage is not necessarily optimally matched to the input of the engine control computer.